Earlier quoted context omitted.
They would probably be sued for being anti competitive if they bought out all the DAW companies and did that.
Apple is not in a monopoly position, so it's a perfectly legal move. As long as they didn't become a monopoly in any specific creative field, it would be fine too.
A literal monopoly is not necessary for a court to determine that a firm can exert significant and durable market power over its competitors and customers.
If a firm is able to achieve and maintain a monopoly solely through lawful and reasonable means, such as producing and offering a superior product at a lower price, it is free to reap the benefits of that fairly-attained monopoly.